1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to disc drive servo control, and more particularly, to quickly and finely compensating for non-repeatable run out (NRRO) of a disc drive.
2. Description of the Related Art
Hard disc drives (HDDs) data storage devices which reproduce data recorded on a disc or write data on a disc using a magnetic head. According to the trend for making high capacity, high density, and compact HDDs, a number of bits per inch (BPI), which denotes a recording density in a disc rotating direction, and a number of tracks per inch (TPI), which denotes the recording density in a disc diameter direction, have been increased. Accordingly, a more elaborate operation mechanism is required for the HDDs.
A purpose of a track following control in an HDD is to place a head on the very center of a target track even if run out exists. The run out is classified into repeatable run out (RRO) generated due to disc eccentricity and non-repeatable run out (NRRO) generated due to disc fluttering. Here, “repeatable” means that a phase of a run out signal is synchronized with a servo sector of a disc. While research related to RRO compensation control has been briskly carried out, research related to NRRO compensation control has not that briskly been carried out due to technical difficulties.
The technical difficulties of the NRRO compensation control can be mainly classified as (1) a frequency of the run out signal varies according to products; and (2) a phase of the run out signal cannot be synchronized with the disc servo sector. It is difficult to develop a quick and fine NRRO compensation controller because of these two eigen characteristics of the NRRO.
When a rotational speed of a disc is increased to improve an operational speed of the HDD, a disc fluttering effect is severely generated, the NRRO is increased, and a disc servo performance is degraded.
U.S. Pat. No. 5,072,318 discloses a method of compensating for the NRRO by estimating a frequency, amplitude, and phase of the NRRO from a position error signal (PES) as described below.
An NRRO value XNRRO(n) of an HDD can be represented as follows in Equation 1.xNRRO(n)=A(n)cos(ω(n)nT+φ(n))  (1)
To compensate for the NRRO, an estimating equation represented as follows in Equation 2 is used.a(n+1)=a(n)+μ cos(ω(n)nT)xPES(n)b(n+1)=b(n)+μ sin(ω(n)nT)xPES(n)ω(n+1)=ω(n)+μnT[b(n)cos(ω(n)nT)−a(n)sin(ω(n)nT)]xPES(n)  (2)
A compensation value according to the estimation result is represented as follows in Equation 3.u(n)=a(n)cos(ω(n)nT)+b(n)sin(ω(n)nT)   (3)
According to the conventional technology, since a frequency value ω(n) is estimated in the estimating equation, estimation stability and convergence speed are degraded. Also, since a trigonometric function value sin(ω(n)nT) of an estimated frequency is necessary to compensate for the NRRO, an exact realization is difficult.
U.S. Pat. No. 6,636,376 discloses a method for compensating for the NRRO by canceling a resonance frequency of a disc drive. In particular, U.S. Pat. No. 6,636,376 discloses a method of estimating a frequency in off-line using discrete Fourier transform (DFT) and using the estimation result in an NRRO compensator. However, when the frequency varies along time, performance is degraded. In addition, when the NRRO compensation is realized, calculation of the trigonometric function is still necessary.